Waste water from acidic hot springs in volcanic regions, acidic mine effluent and acidic underground water in regions of volcanic soil contain sulfuric acid formed by the oxidation of sulfur-containing substances and iron sulfide ores. such acidic waste water exerts an adverse influence on concrete structures such as bridges and dams and, still more, acids and heavy metals such as iron and arsenic therein contained degrade water quality, exterminate fish and shellfishes and cause the so-called “red river” phenomenon when discharged indiscriminately. For this reason, it is necessary to submit acidic waste water to a neutralization treatment.
A method in wide use for this neutralization treatment consists of adding particles or a slurry of slaked lime to waste water. The chemical substance used in this method is relatively low in cost and has an excellent ability to neutralize acidic waste water; however, in the cases where waste water contains sulfate ions and iron ions in large quantities, the iron ions precipitate taking the form of colloidal ferric hydroxide as the pH rises and, besides, the sulfate ions react with slaked lime to form difficultly soluble gypsum and precipitate together with some of the unreacted slaked lime to form a slimy matter that is high in water content and difficult to dewater. While this is happening, heavy metals such as arsenic in waste water are adsorbed on the ferric hydroxide and precipitate together. As this slime is a highly hydrous slurry that is difficult to dewater and contains harmful substances, its disposal requires installation of a solid-liquid separator such as an expensive thickener, a sedimentation basin and a device for dewatering and compacting slime such as a labor-consuming filter press and construction of a dam for accumulating slime as a final disposal device. Thus, the disposal of slime poses problems of increased disposal cost and harmful influences on natural environment.
In order to reduce formation of slime that is highly hydrous and difficult to dewater, the use of magnesium oxide particles as a neutralizing material has been studied because magnesium oxide forms slime that is easier to dewater and does not form difficultly soluble reaction products such as gypsum, but a high cost of the chemical in question is a disadvantage.
Furthermore, in order to reduce cost and improve the dewatering performance of slime, the use of calcium carbonate particles and limestone grains has been tried. However, gypsum resulting from neutralization covers the surface of calcium carbonate or limestone to hinder a further progress of neutralization thereby reducing the efficiency in utilization of the neutralizing material. A neutralizing material based on calcium carbonate produces only a small effect for raising the pH and cannot precipitate ferrous ions in waste water as ferrous hydroxide. Therefore, a pretreatment becomes necessary to oxidize ferrous ions to ferric ions by aeration or by iron-oxidizing bacteria.
Application of inorganic fibers to a treatment of waste water as a filtering material or a material for binding microorganisms is disclosed in JP6-315681A and elsewhere but nothing is taught of the use of inorganic fibers as a material for neutralizing acidic waste water.
JP2000-73347A discloses drainage materials composed of inorganic fibers and inorganic hydraulic materials for underdrainage, but they are regarded as substitutes for the husks of rice used up to the present.